Nuclear reactors



March 12, 1963 P. J. GRANT 3,081,248

NUCLEAR REACTORS Filed March 25, 1959 2 sheets-sheet 1 .1N VEN 'rank FlTTORNEYS 2 Sheets-Sheet 2 Filed March 26, 1959 llli Yatented Mar. i2,1963 i u ii@ 3,081,248 NUCLEAR REACTGRS Peter John Grant, Longfield,England, assigner to The General Electric Company Limited, London,England Filed Mar. 26, 1959, Ser. No. 802,061 Claims priority,application Great Britain Mar. 31, i958 Claims. (Cl. 20d-193.2)

The present invention relates to nuclear reactors of the kind having amoderator associated with, or adapted to be associated with, ssiblematerial.

The invention is applicable, for example, to nuclear reactors of thekind having a moderating core in which fuel elements are adapted to belocated. The neutron flux in the core of such a reactor varies from thecentre of the core to its boundary, and if the core is of the typehaving axial symmetry, as with a cylindrical core, the neutron iluxvaries in the axial direction from a minimum at each end to a maximum atthe centre.

Some of the fission products formed in such reactors, particularly Xe135have a very high capture cross section for slow neutrons, and these tendto have a long term stabilising effect on the neturon ilux distributionsince if there is an increase in the flux in one part of the core thereis an increase in the amount of Xe135 produced and this tends to reducethe iiux again locally. The total flux is controlled by retarders, orcontrol rods, which are inserted into the core axially.

However, in larger reactors than those constructed hitherto there is alikelihood that an unstable, and possibly oscillatory, distortion of theneutron ux in the axial direction will occur since an increase inneutron flux leads to the burn-out of Xel35 (produced by radioactivedecay of 11351), releasing reactivity which leads to a further fluxdistortion and so on. ln the course of time the concentration of Xe135returns to equilibrium with the new ux level, when the whole process isreversed. The likelihood of a ilux distortion is increased if there is apositive coeiiicient of reactivity associated with either fuel ormoderator temperature. This distortion takes the form of an increase ofilux towards one end of the reactor and a decrease of ilux towards theother end.

It is an object of the present invention to provide means for correctingsuch flux distortion in a reactor core.

According to one aspect of the present invention, in a nuclear reactorof the kind having a moderator associated with, or adapted to ybeassociated with, fissile material, means are provided for controllingthe distribution of the neutron ilux in the moderator, said meanscornprising a pair of neutron absorber members adapted to be insertedinto diierent parts of the moderator and linked or coupled wherebymovement of one member into, or out of, the moderator is accompanied bya corresponding movement of the other member out of, or into, themoderator, the arrangement being such that by the differential movementof the two members the uxes in the said two parts of the moderator can:be relatively adjusted without substantially affecting the total ux.

According to another aspect of the present invention, in a nuclearreactor of the kind having a moderating core in which fuel elements areadapted to be located, means are provided for controlling thedistribution of the neutron flux in the core, said means comprising apair of neutron absorber members adapted to be inserted into differentparts of the core and linked or coupled Whereby movement of one memberinto, or `out of, the core is accompanied by a corresponding movement ofthe other member out of, or into, the core, the arrangement being suchthat by the dilerential movement of the two members the uxes in the saidtwo parts of the core can =be elatively adjusted without substantiallyaiecting the total ux.

Thus the said means may comprise a control rod extending through anaxial passage in the moderating core, the rod having two end sections ofneutron-absorbing material connected by an intermediate section which isrelatively nonabsorbent to neutrons.

Two arrangements in accordance with the present invention will now bedescribed, by way of example, with references to the accompanyingschematic drawings, in which each ligure shows, in section, afragmentary view of a moderator core and one means for controlling ltheilux distribution .in the core.

Referring to FIGURE l of the drawings, a nuclear reactor comprises amoderating core 1 in which fuel elements (not shown) are located, thecore being cylindrical and having passages such as 2 extendingtherethrough parallel to the axis. The core is mounted on a supportstructure 3 within a pressure vessel 4, the pressure vessel beingsupported on an annular skirt 5 connected to a base 6. Pipes 7 forcirculating gaseous cooling medium under pressure are welded into thevessel 4.

ln order to correct for distortion of the neutron ilux in the core inthe axial direction, which distortion would take the form of a iiuxincrease towards one end of the core relatively to the other end, acontrol rod 13 extends absorbent to neutrons, such as boron, cadmium orgadolinium, an end section 9 of the same material, and an intermediatesection 10 connecting the two end sections, the section 10 beingrelatively nonabsorbent to neutrons. Each of the end sections 8 and 9 isnormally disposed adjacent to one end of the passage 2 and is adapted tobe'inserted into the core at that end so as to act as a neutron absorbermember in the manner of a conventional control rod. It will be seenhowever that movement of one section into the core is accompanied by acorresponding movement of the other member' out of the core so that theflux at the two ends of the core is controlled differentially bymovement of the rod 13, the relative flux o-f the two ends being variedwhile the total ilux remains substantially constant.

In order to effect such movement of the control rod, the rod issuspended by a cable 11 from an electric motordriven winch 12. It is tobe understood, of course, that in practice 4there will be many suchcontrol rods each cooperating with a passage such as 2 and arranged inthe manner of the control rod 13.

When the ilux distribution is symmetrical, that is when there is no uxdistortion, the two neutron absorber members would be adjacent to theends of the passage 2 as -shown in the drawing; but if the flux ltowardsone end of the core should increase while the iiux towards the other enddecreases, the rod 13 would be moved so as to insert the neutronabsor-ber member at the one end farther into the passage 2 in the corewhereby the increased neutron flux towards that end would be reduced,and simultaneously the other retarder member would be withdrawn fromlthe passage to a corresponding extent thus enabling Ithe decreasedneutron ilux at that end to build up.

Referring now lto FIGURE 2, which illustrates a modiiied arrangement,the neutron absorber members 8 and 9 are arranged to cooperate with twoseparate passages 14 and 15 in t-he core 1. The retarder member 18 issupported by a cable 16 from an electric motor driven winch 17, and theretarder member 9 is connected -to a section 18 of material which isrelatively non-absorbent to neutrons, this section being supported by acable 19 from an electric motor driven winch 20. The Winches y19' and 20may be mounted on a common shaft and driven by the one electric motor,but in the arrangement illustrated the winches are separately driven,their respective motors being controlled by a common control device 21operative to control the motors in such a manner that any movement ofone retarder member in its passage is accompanied by a correspondingmovement of the other member such that there is no net change of neutronilux in the core.

It will -be seen that with the arrangements described above, theover-all reactivity change due to movement of the control Iis smallsince movement of the two neutron absorber members with respect to thecore is made differentially; moreover it is never necessary to move thecontrol rod through large distances. On the other hand, if a control rodof conventional construction were to ybe used for correcting fluxdistortion, it would be necessary to be above to move the rod from oneend of the core to the other and such movement of the rod through thecore would seriously aect the total reactivity.

I claim:

1. In a nuclear reactor having an active portion comprising iissilematerial and moderating mater-ial, said ac- Itive portion having an axisof symmetry and the volume of said active portion being such that theneutron flux can be asymmetrically distributed between the 4two outerantipodal regions of `said active portion in the direction of said axisin a manner such that there is an increase in reactivity at one suchouter region and a corresponding decrease in reactivity at the antipodalregion, means for correcting such asymmetrical flux distribution, saidlast-named means comprising means providing openings in said antipodalregions and a pair of neutron absorber members extending into saidopenings of the active portion respectively, and motive means coupledwith said members -in a manner to move said members simultaneously intoand out of said openings in said antipodal regions respectively.

2. In a nuclear reactor having an active portion cornprising amoderating core and fuel elements located in said core, said activeportion having an axis of symmetry and the volume of said active portionbeing such that the neutron flux can be asymmetrically distributedbetween the two outer antipodal regions of said active portion in thedirection of said axis in a manner such that there is Van increase inreactivity at one such outer region and a corresponding decrease inreactivity at the antipodal region, means for correcting suchasymmetrical ux d-istribution, said last-named means comprising meansproviding openings in sai-d antipodal regions and a pair of neutronabsrober members extending into said openings of the active portion,respectively, and motive means coupled with said members in a manner tomove said members simultaneously into and out of said openings in saidantipodal regions respectively.

3. In a nuclear reactor having an active portion comprising a moderatingcore and fuel elements located in said core, said active portion havingan axis of symmetry and the volume of said active portion being suchthat the neutron. tiux can be asymmetrically distributed between the tWoouter antipodal regions of said active portion in the direction of saidaxis in a manner such lthat there is an increase in reactivity at onesuch outer region and a corresponding decrease in reactivity at theantipodal region, means for correcting such asymmetrical iluxdistribution, said last-named means comprising means providing parallelpassages lin said active por-tion and extending through said antipodalregions, control rods within said passages, and motive means for movingsaid control rods along said passages, each said control rod cornprisinga pair of end sections of neutron-absorbing material cooperating withthe ends of one of said passages, and an intermediate section connectingsaid end sections, said intermediate section being relativelynonabsorbent lto neutrons.

4. In a nuclear reactor having an active portion cornprising amoderating core and fuel elements located in said core, said activeportion having an axis of symmetry and the volume of said active portionbeing such that the neutron flux can be asymmetrically distributed be-Itween the two outer antipodal regions of said active portion in thedirection of said axis in a manner such that there is an increase inreactivity at one such outer region and a corresponding decrease inreactivi-ty at the antipodal region, means for correcting suchasymmetrical ilux distribution, said last-named means comprising meansproviding pairs of parallel passages in said active portion extendingthrough said antipodal regions, pairs of neutron absorber memberscooperating respectively with said passages adjacent opposite sides ofsaid active portion, and motive means coupled with said members in amanner to move the members simultaneously into and out of said activeportion respectively.

5. `In a nuclear reactor having an active portion comprising acylindrical core of moderating material and fuel elements located insaid core, said active portion having an axis of symmetry and the volumeof said active portion being such that the neutron flux can beasymmetrically distributed between the two outer antipodal regions ofsaid active portion in the direction of said axis rin a maner such thatthere is an increase in reactivity at one such outer region and acorresponding decrease in reactivity at the antipodal region, means forcorrecting such asymmetrical flux distribution, said last-named meanscomprising means providing openings in said antipodal regions in adirection parallel to the axis of said active portion and a pair ofneutron absorber members extending into said openings, and motive meanscoupled with said members in a manner to move said memberssimultaneously into and out of the respective openings.

References Cited in the le of this patent UNITED STATES PATENTS

1. IN A NUCLEAR HAVING AN ACTIVE PORTION COMPRISING FISSILE MATERIAL ANDMODERATING MATERIAL, SAID ACTIVE PORTION HAVING AN AXIS OF SYMMETRY ANDTHE VOLUME OF SAID ACTIVE PORTION BEING SUCH THAT THE NEUTRON FLUX CANBE ASYMMETRICALLY DISTRIBUTED BETWEEN THE TWO OUTER ANTIPODAL REGIONS OFSAID ACTIVE PORTION IN THE DIRECTION OF SAID AXIS IN A MANNER SUCH THATTHERE IS AN INCREASE IN REACTIVITY AT ONE SUCH OUTER REGION AND ACORRESPONDING DECREASE IN REACTIVITY AT THE ANTIPODAL REGION, MEANS FORCORRECTING SUCH ASYMMETRICAL FLUX DISTRIBUTION, SAID LAST-NAMED MEANSCOMPRISING MEANS PROVIDING OPENINGS IN SAID ANTIPODAL REGIONS AND A PAIROF NEUTRON ABSORBER MEMBERS EXTENDING INTO SAID OPENINGS OF THE ACTIVEPORTION RESPECTIVELY, AND MOTIVE MEANS COUPLED WITH SAID MEMBERS IN AMANNER TO MOVE SAID MEMBERS SIMULTANEOUSLY INTO AND OUT OF SAID OPENINGSIN SAID ANTIPODAL REGIONS RESPECTIVELY.